The recovery of olefins such as ethylene and propylene from gas mixtures is an economically important but highly energy intensive process in the petrochemical industry. These gas mixtures are produced by hydrocarbon pyrolysis in the presence of steam, commonly termed thermal cracking, or can be obtained as offgas from fluid catalytic cracking and fluid coking processes. Cryogenic separation methods are commonly used for recovering these olefins and require large amounts of refrigeration at low temperatures.
Olefins are recovered by condensation and fractionation from feed gas mixtures which contain various concentrations of hydrogen, methane, ethane, ethylene, propane, propylene, and minor amounts of higher hydrocarbons, nitrogen, and other trace components. Methods for condensing and fractionating these olefin-containing feed gas mixtures are well-known in the art. Refrigeration for condensing and fractionation is commonly provided at successively lower temperature levels by ambient cooling water, closed cycle propylene and ethylene systems, and work expansion or Joule-Thomson expansion of pressurized light gases produced in the separation process. Recent improvements in cryogenic olefin recovery methods have reduced energy requirements and increased recovery levels of ethylene and/or propylene.
Many methods have been proposed to provide refrigeration to cryogenic separation processes for the recovery of C.sub.2 or C.sub.3 and heavier hydrocarbons. These methods include work expansion of the feed gas or the light residue gas, conventional single-fluid or cascade vapor compression refrigeration, mixed refrigerant, and Joule-Thomson expansion refrigeration. Other processes utilize absorption for the recovery of C.sub.2 or C.sub.3 and heavier hydrocarbons, which typically reduces the amount of refrigeration required for the separation process.
U.S. Pat. Nos. 5,568,737, 5,555,748 and 4,752,312 describe processes utilizing work expansion of the feed gas to provide refrigeration for recovery of C.sub.2.sup.+ or C.sub.3.sup.+ hydrocarbons from natural gas or refinery gas streams. U.S. Pat. Nos. 5,275,005, 4,895,584 and 4,617,039 describe similar processes where a conventional propane or other vapor recompression refrigeration system is used to supplement the refrigeration provided by work expansion of the feed gas. These processes require relatively high feed gas pressure, typically 500 to 1000 psia, and relatively low C.sub.2 content in the feed in order to provide sufficient refrigeration for high C.sub.2 recovery (90% or more). They are generally more suitable for C.sub.3 recovery which requires warmer refrigeration than that required for C.sub.2 recovery. U.S. Pat. No. 4,714,487 describes a similar process utilizing work expansion of the light residue gas to provide refrigeration for recovery of C.sub.3.sup.+ hydrocarbons.
A conventional cascade vapor compression refrigeration system is disclosed in U.S. Pat. No. 5,502,971 which utilizes an ethylene/propylene system to provide refrigeration for recovery of C.sub.2.sup.+ hydrocarbons from a refinery off-gas stream. This type of refrigeration is used in essentially all ethylene plants to recover ethylene and heavier hydrocarbons from cracked gas. This type of cascade system can provide refrigeration efficiently at temperature levels as low as -150.degree. F. but requires two refrigerant compressors and multiple refrigerant drums.
Joule-Thomson expansion and revaporization of separated C.sub.2.sup.+ hydrocarbons to provide refrigeration for recovery of those hydrocarbons from a cracked gas is described in U.S. Pat. No. 5,461,870. This process is energy efficient but requires that the hydrocarbon product be recovered as a vapor at relatively low pressure in order to provide refrigeration at the low temperature level that is necessary for the separation.
U.S. Pat. Nos. 5,329,779, 5,287,703, 4,707,170 and 4,584,006 utilize various forms of mixed refrigerant systems to provide refrigeration for recovery of C.sub.2 or heavier hydrocarbons from various hydrocarbon containing streams. These processes utilize a single refrigerant compressor to provide refrigeration over a wide temperature range but require multiple refrigerant drums and complex refrigerant make-up systems.
Processes utilizing absorption for the recovery of C.sub.2.sup.+ or C.sub.3.sup.+ hydrocarbons from cracked gas, refinery gas, or natural gas is disclosed in U.S. Pat. Nos. 5,520,724, 5,019,143 and 4,272,269. The light hydrocarbons are absorbed in a heavier solvent, usually a C.sub.5 or heavier hydrocarbon, in an absorption column and stripped in a separate column to recover the light product and regenerate the heavy solvent. Conventional vapor recompression refrigeration is usually required to refrigerate the solvent, typically to about -40.degree. F., in order to achieve high C.sub.2 recovery.
Nitrogen recycle refrigeration systems have been used in cryogenic air separation plants to provide very low temperature refrigeration (-280 to -320.degree. F.) for the production of liquid oxygen and liquid nitrogen products (see U.S. Pat. Nos. 5,231,835, 4,894,076, and 3,358,460). Nitrogen recycle refrigeration systems have not been used, however, for C.sub.2 and C.sub.3 hydrocarbon recovery at warmer temperatures (-50 to -250.degree. F.).
The cryogenic separation methods described above for recovering C.sub.2.sup.+ and C.sub.3.sup.+ hydrocarbons require large amounts of refrigeration at low temperatures. It is desirable to reduce the energy consumed for these refrigeration requirements by utilizing new or improved refrigeration processes which can be installed at reasonable capital cost. The process of the present invention, which is described below and defined by the claims which follow, utilizes a low-cost and energy-efficient method to supply such refrigeration.